Diamond is a material with high hardness, corrosion resistance, very small isotropic magnetic susceptibility, and ultra-wide transparency range. These unique properties lead to extensive applications in various fields. [1], [2], [3], [4] The transition of diamond to graphite can happen when heated up to 2000 K in a vacuum chamber at zero pressure [5]. Many experimental researches have shown that pulse laser can also lead to this phase transition. In these researches, the laser parameters are very various, duration from nanosecond (ns) pulse [6], picosecond (ps) pulse [7] to femtosecond (fs) pulse [2], [8], and wavelength from ultraviolet (UV) [9], visible [10] to near-infrared [2], [8], [11]. A lot of pure carbon devices depending on diamond–graphite phase transition have been microfabricated by employing a fs laser direct writing with different wavelengths [2], [4], [8], [9], [11] since fs laser is a powerful tool for three-dimensional micromachining due to its extremely high peak intensity and ultrashort duration.

The dynamics of ultrashort pulse laser induced diamond phase transition have been studied in detail when the sample was irradiated by ps and fs pulses with a repetition rate of less than 1 kHz [12], [13], [14], [15]. The experimental results showed that there are two types of dynamics of the phase transitions: the photoinduced graphitization and the thermal graphitization [13]. Initially, the graphitized nano-region is generated and its size grows due to direct photoinduced graphitization. The diamond–graphite phase transition is abrupt and the conductivity of the graphitized region is at least several orders of magnitude higher than that of diamond, so that the graphitized micro-region can linearly absorb the laser energy because the material becomes semi-metal. Hence, the thermal graphitization takes place, and the volume of this graphitized micro-region fast increases toward the laser beam [13]. A model of crack-assisted thermal graphitization of diamond was introduced for explaining the growth dynamics [14]. However, to our knowledge, few paper has reported the dynamics of the graphitization caused by a high repetition rate fs laser in the case of fast scanning along the direction perpendicular to the beam.

In this paper, we studied the dynamics of the graphitization by employing 100 kHz fs laser. The experimental results show that a fully continuous graphitization filament toward the beam can be fabricated at an energy density very close to the threshold while the laser beam is focused inside the sample. Unlike the dynamics in the case of employing low repetition rate fs laser, the graphitized region can grow toward the laser beam (i.e. toward the front surface of sample) and along the beam (i.e. toward the rear surface). A thin graphitization wire can be easily fabricated by moving the focus on the front surface of the sample. A multi-scanning can decrease the fluctuations of the wire thicknesses in the laser propagation direction in the surface processing, but it is useless to the profile of the graphitization wire when scanning inside the sample. Along the laser scanning direction, rather than a continuous graphitization wire, a periodic microstructure is fabricated. It looks like a comb, consisting of an array of the graphitization filaments along the beam propagation direction. But unlike the straight filament when focusing the laser at an internal fixed position, these filaments are straight at the root and curved at the top. This configuration can be explained by four growth processes. In nature, the emergence of such a comb microstructure hinders writing a continuous wire with a high conductivity by simply scanning whatever the processing parameters are.

In addition, we investigated the magnetic properties of the irradiated samples. The results show that there are large differences between the surface and internal graphitized sample. The internal graphitized sample has not only an obvious anisotropy of magnetic susceptibility but also an unusual superparamagnetism. The data of the surface graphitized sample illustrates its diamagnetism with isotropic susceptibility. The reason of this superparamagnetism must be very complicated and its characteristics needs to be explored further.